Excited-State Modulation in Donor-Substituted Multiresonant Thermally Activated Delayed Fluorescence Emitters

Strategies to tune the emission of multiresonant thermally activated delayed fluorescence (MR-TADF) emitters remain rare. Here, we explore the effect of donor substitution about a MR-TADF core on the emission energy and the nature of the excited state. We decorate different numbers and types of electron-donors about a central MR-TADF core, DiKTa. Depending on the identity and number of donor groups, the excited state either remains short-range charge transfer (SRCT) and thus characteristic of an MR-TADF emitter or becomes a long-range charge transfer (LRCT) that is typically observed in donor–acceptor TADF emitters. The impact is that in three examples that emit from a SRCT state, Cz-DiKTa, Cz-Ph-DiKTa, and 3Cz-DiKTa, the emission remains narrow, while in four examples that emit via a LRCT state, TMCz-DiKTa, DMAC-DiKTa, 3TMCz-DiKTa, and 3DMAC-DiKTa, the emission broadens significantly. Through this strategy, the organic light-emitting diodes fabricated with the three MR-TADF emitters show maximum electroluminescence emission wavelengths, λEL, of 511, 492, and 547 nm with moderate full width at half-maxima (fwhm) of 62, 61, and 54 nm, respectively. Importantly, each of these devices show high maximum external quantum efficiencies (EQEmax) of 24.4, 23.0, and 24.4%, which are among the highest reported with ketone-based MR-TADF emitters. OLEDs with D–A type emitters, DMAC-DiKTa and TMCz-DiKTa, also show high efficiencies, with EQEmax of 23.8 and 20.2%, but accompanied by broad emission at λEL of 549 and 527 nm, respectively. Notably, the DMAC-DiKTa-based OLED shows very small efficiency roll-off, and its EQE remains 18.5% at 1000 cd m–2. Therefore, this work demonstrates that manipulating the nature and numbers of donor groups decorating a central MR-TADF core is a promising strategy for both red-shifting the emission and improving the performance of the OLEDs.


■ INTRODUCTION
Thermally activated delayed fluorescence (TADF) materials are a promising class of emitters for organic light-emitting diodes (OLEDs) as the devices can realize up to 100% internal quantum efficiency, while the organic emitters can be easily synthesized at a low cost and are sustainable. 1−3 TADF operates by converting nonemissive triplet excitons into singlets through endothermic reverse intersystem crossing (RISC). RISC between singlet and triplet excited states is only possible when there is spin−orbit coupling (SOC) between them, and when the energy gap between them (ΔE ST ) is sufficiently small. The magnitude of ΔE ST is correlated with the degree of overlap of the orbitals involved in the transition, which typically are the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). 4 The corresponding molecular design usually involves minimizing the conjugation between the electrondonating moieties and electron-accepting moieties by adopting a strongly twisted conformation between these two units. This commonly used design has a number of drawbacks. Due to the large redistribution of the electron density during the transition, the nature of the emission is charge transfer (CT) between the donor and acceptor. 5 This, coupled with the conformational flexibility inherent in the design, leads to a broad emission spectrum (full width half-maximum, fwhm, of 80−200 nm). 6 This significantly degrades the color gamut of the OLEDs, which is an undesirable trait for displays. Therefore, there is at present a growing effort to develop TADF materials that show both a small ΔE ST and narrow emission spectra.
One subclass of TADF emitters that responds to these criteria are multiresonant TADF emitters (MR-TADF). Examples of MR-TADF emitters are shown in Figure S3. MR-TADF emitters are nanographenes containing suitably positioned electron-donating atoms (e.g., N, O, and S) and electron-deficient atoms or groups (e.g., B and CO) within the fused aromatic framework. In these compounds, electrons and holes are localized on adjacent atoms due to the complementary mesomeric effect of the electron-donating and electron-accepting units leading to the required small exchange integral and ΔE ST . 7 This electron density distribution is reflected in the small degree of positive solvatochromism that is characteristic of a short-range charge transfer (SRCT) excited state. Although nearly 200 MR-TADF emitters have been reported since the first example in 2016, 8 the majority show blue or green emission. There are still very few examples that emit at longer wavelengths. 7 Therefore, this work focuses on developing longer wavelength MR-TADF emitters.
There are several potential strategies that may be employed to tune the emission energy toward the red. The first involves strategic placement of the relative positions of the electrondonating and electron-accepting groups. In 2020, Yasuda and co-workers, 9 arranged two electron-donating nitrogen atoms para to each other and two boron atoms para to each other about the central phenyl ring (namely, B-π-B and N-π-N), which led to a significantly red-shifted emission and is the first example of a red MR-TADF emitter, BBCz-R, showing an emission maximum, λ PL , of 615 nm in toluene solution. Duan and co-workers, 10 adopted a similar strategy, producing two red emitters, R-BN and R-TBN, with N-π-N arrangements (λ PL = 662 and 692 nm in toluene solution). A second strategy involves modifying the MR-TADF core with electron-donating groups to increase the charge transfer character. Hatakeyama and co-workers 11 reported a green emitter (λ PL of 506 nm in 1 wt % PMMA film), OAB-ABP-1, that contains an extended πskeleton that consists of an alternating pattern of para-disposed O−B−N atoms. Kido and co-workers 12 reported a green emitter PXZ-BN with λ PL of 502 nm by replacement of carbazole for phenoxazine within BCz-BN skeleton. In a similar vein, Yang and co-workers incorporated sulfur, affording the green emitter 2PTZBN (λ PL = 510 nm in toluene), with the expectation of enhancing spin−orbit coupling and hence the reverse intersystem crossing rate. 13 A third strategy to modulate the emission involves the incorporation of peripheral electron-donating or electronaccepting moieties. Duan and co-workers 14 reported the first examples of green-emitting MR-TADF emitters (λ PL = 502 nm in 6 wt % doped mCPCB 9-(3-(9H-carbazol-9-yl)phenyl)-9H-3,9′-bicarbazole (mCPBC) film), 2F-BN, by decorating peripheral electron-withdrawing fluorophenyl groups para to the central boron atom. Wang and co-workers also employed the same strategy, incorporating electron-withdrawing benzonitrile units in compound (R)-OBN-4CN-BN, obtaining a green emitter (λ PL = 500 nm in toluene). Duan and coworkers 15 reported another green-emitting MR-TADF compound (AZA-BN) that incorporates a fused azaphenanthrene (λ PL = 522 nm in toluene). Using the same BCz-BN skeleton, Wang and co-workers reported the green emitter (m-Cz-BNCz) that contains a meta-disposed auxiliary di-tertbutylcarbazole with respect to the central boron atom (λ PL = 519 nm in toluene). 16 Recently, Yang and co-workers, introduced donor groups at the para position of the carbazole of the BCz-BN skeleton and demonstrated color modulation from sky blue to yellow (λ PL of 496 to 562 nm in toluene). 17 Using the same BCz-BN skeleton, You and co-workers combined both donor and acceptor groups located para to the N and B atoms, respectively, to realize orange emission (λ PL of 581 nm in toluene). 18 Although B/N-based emitters have realized full-color emission, their synthesis can only be reached through lithiation−borylation−cyclization reaction or electrophilic fixed-point C−H borylation cyclization reaction, which complicates downstream elaboration of these structures. A second family of MR-TADF compounds employ electronaccepting carbonyl groups in lieu of boron atoms. We showed that decorating the MR-TADF emitter DiKTa with mesityl groups, Mes 3 DiKTa, can mitigate undesired aggregation caused quenching (ACQ) and excimer emission while also modestly red-shifting the emission (λ PL = 468 nm in toluene). 19 We also reported a dimeric compound, DDiKTa, consisting of two DiKTa units, that showed a red-shifted emission with λ PL of 500 nm. 20 Liao and co-workers, reported structurally rigid analogs of DiKTa that incorporated a carbon-, oxygen-, or sulfur-based tether. The compounds DQAO, OQAO and SQAO showed red-shifted emission compared to that of DiKTa with λ PL ranging from 465 to 552 nm in toluene. 21 Zhang and co-workers, reported the compounds QAD-Cz, QAD-2Cz and QAD-mTPDA that contain donor groups decorating the DiKTa core to afford D− A type emitters. These molecules showed blue to red emission, with λ PL in the range of 488−586 nm in toluene solution. 22 In this context, we decorated the DiKTa core with different numbers of donors with differing electron-donating strengths. These donors include carbazole (Cz), 9,9-dimethylacridan (DMAC), carbazolyl-phenyl (Cz-Ph), and 1,3,6,8-tetramethyl-9H-carbazole (TMCz). These donors were positioned para to the central electron-donating nitrogen atom. We thus built a framework to systematically study the impact on the emission color and nature of the excited state of the inclusion of these electron-donating groups ( Figure 1). It was found that by introducing weak donors such as carbazole to the para-carbon position of nitrogen the HOMO levels of the new emitters were significantly destabilized compared to that of parent DiKTa, while the LUMO levels were barely perturbed, leading to the desired red-shifted emission. Importantly, the narrow emission characteristic of MR-TADF emitters was maintained. However, when the electron-donating strength was further increased, the emission nature changed to long-range charge transfer (LRCT) and the emission spectra significantly broadened. In this study, weaker-donor-based emitters, Cz-DiKTa, 3Cz-DiKTa, and Cz-Ph-DiKTa, maintained their SRCT character in all the tested environments, while strongerdonor-based emitters, TMCz-DiKTa, DMAC-DiKTa, 3TMCz-DiKTa, and 3DMAC-DiKTa, showed a more complicated behavior where LRCT emission dominates in polar media.
Theoretical Studies. The frontier molecular orbitals (FMOs) of these emitters were first modeled based on the optimized ground state gas-phase geometry using density functional theory (DFT) at the PBE0/6-31G(d,p) level of theory. The HOMO and LUMO distributions are shown in Figure S5, and the HOMO and LUMO level of the seven compounds are listed in the Table S1. Compared to DiKTa (−5.94/−2.31 eV), the HOMO level is destabilized by 0.21− 0.68 eV, and the degree of destabilization correlates with both the strength and numbers of the peripheral donor group. Within the series of the donor-substituted DiKTa compounds, we noticed that the occupied orbitals localized on the DiKTa core do not correspond to the HOMO, instead, their HOMOs reside on the donor (see Figure S5). For the compounds with the weakest donors (Cz-DiKTa, Cz-Ph-DiKTa, and 3Cz-DiKTa) this orbital slightly delocalizes to the carbazoles, thereby resulting in destabilization compared to DiKTa while for the compounds containing the strongest donors (TMCz-DiKTa, DMAC-DiKTa, and 3DMCz-DiKTa) the energy of the DiKTa-localized orbital remains unaffected.
We employed spin component scaled second order approximate coupled cluster (SCS-CC2) with the cc-pVDZ basis set to more accurately model the nature of the charge transfer excited states. Figure 2 shows the difference density plots of the S 1 and the second lowest singlet excited state (S 2 ) transitions of these emitters obtained using SCS-CC2. Cz-DiKTa, Cz-Ph-DiKTa, and 3Cz-DiKTa all exhibit similar S 1 difference density plot patterns akin to that for DiKTa (shown in Figure S7). On the basis of the charge-transfer distance, D CT < 1.4 Å, we assign these excited states to be SRCT (Table S2). At the same time, small contributions to the difference density plots can be seen at the peripheral donor fragments in the new emitters, especially on the "top" carbazole moiety in 3Cz-DiKTa. The S 2 state of each of Cz-DiKTa, TMCz-DiKTa, and Cz-Ph-DiKTa possesses n−π* character, while for 3Cz-DiKTa S 2 remains a π−π* transition. The S 1 difference density plot of DMAC-DiKTa is almost identical to that of DiKTa, while the S 2 difference density plots show LRCT character as the increased density can be seen on the electrondeficient DiKTa core and the decreased density is located on the peripheral DMAC moieties. Indeed, the D CT of this state is 3.18 Å, which is characteristic of a LRCT state. Since all the calculations are carried out in gas-phase, potentially the nature of the emissive excited state may switch between SRCT and LRCT, depending on the environment as the energy gap between the S 1 and S 2 (ΔE S1S2 ) of this emitter is small (0.34 eV) and the S 2 electrical dipole moment for the S 2 state is large. We have also carried out the same calculations on other reported donor−acceptor type emitters containing an MR-TADF moiety acting as the acceptor to validate our computational methodology ( Figure S4). 23 All the investigated emitters show SRCT S 1 states in the gas-phase calculations and a narrow emission characteristic of a MR-TADF behavior. ADBAN-Me-MesCz and DABNA-2 show large ΔE S1S2 of 0.71 and 0.64 eV at the SCS-CC2 level, which suggests that this ΔE S1S2 energy gap is large enough to prevent the switching of light emission from the SRCT state to the LRCT state when considering the impact of solvent effects or polarization effects arising in the solid state. By contrast, PXZ-DABOA, TDBAAc, TDBA-DI and QAO-dic all have predicted LRCT S 2 states , and much smaller ΔE S1S2 values of 0.04−0.29 eV. Such a small energy difference implies that LRCT-SRCT inversion is possible when the medium is sufficiently polar. In the case of 3DMCz-DiKTa and 3DMAC-DiKTa, the S 1 states show a strong contribution of the hole density on the top Cz unit, while the electron density is mainly localized on the DiKTa Optoelectronic Properties. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) measurements were used to experimentally determine the HOMO and LUMO levels. The CV and DPV profiles in dichloromethane are shown in Figure 3a (3TMCz-DikTa and 3DMAC-Dikta are shown in Figure S8), and the electrochemical data are summarized in Table S3. The CV profile of Cz-DiKTa, Cz-Ph-DiKTa, TMCz-DiKTa, DMAC-DiKTa, 3Cz-DiKTa, 3TMCz-DiKTa, and 3DMAC-DiKTa all show reversible reduction waves, which corresponds to the reduction localized on the DiKTa core. While Cz-DiKTa and Cz-Ph-DiKTa show irreversible oxidation waves, which are assigned to the oxidation of the carbazole, when the donor is DMAC and TMCz, the oxidation waves become significantly more reversible. The E red /E ox values of all seven emitters are determined from the peak of the DPVs. The LUMO levels of Cz-DiKTa, Cz-Ph-DiKTa, TMCz-DiKTa, and DMAC-DiKTa are almost identical to that of DiKTa (HOMO/ LUMO values of −6.12/−3.00 eV). When the numbers of donors are increased (3Cz-DiKTa, 3TMCz-DiKTa, and 3DMAC-DiKTa), the LUMO stabilizes by ca. 0.2 eV. In comparison, increasing the electron-donating strength of the peripheral donor (e.g., from Cz to DMAC) results in shallower HOMO levels. Similarly, increasing the number of electron donors also results in shallower HOMOs. Therefore, both strategies can be used to reduce the HOMO−LUMO band gap.
The room-temperature ultraviolet−visible (UV−vis) absorption, steady-state photoluminescence (PL), recorded at roomtemperature (SS RT) and PL spectra of the prompt and delayed emission recorded at 77 K (the latter being the phosphorescence spectra) in toluene (10 −5 M) are shown in Figure 3b−f, and Figure S9 shows the PL spectra for 3TMCz-DiKTa and 3DMAC-DiKta. The corresponding data are summarized in Table S4. The absorption spectra all show two characteristic absorption bands. The higher energy bands (300−430 nm) are attributed to π−π* locally excited (LE) transitions of both the donors and the DiKTa core, and the lower energy bands between 443 and 492 nm are attributed to SRCT transitions centered on the DiKTa core that are characteristic of MR-TADF emitters. Compared to DiKTa, 19 the lowest-energy absorption band of the Cz-DiKTa, Cz-Ph-DiKTa and 3Cz-DiKTa progressively red-shifts and becomes broader and weaker as shown in Figure 3, suggesting the increasing CT character of the transitions associated with this band. The CT absorption bands of TMCz-DiKTa and DMAc-DiKTa have lower molar absorptivities, which can be rationalized by the small FMO overlap due to the strongly twisted conformation of the bulky TMCz donor and the electron-donating strength of the DMAC donor. The seven compounds show a progressively red-shifted emission with increasing number and strength of the electron donors from 472 to 667 nm compared with those of DiKTa (λ PL = 460 nm, fwhm = 27 nm). Cz-DiKTa, Cz-Ph-DiKTa, and 3Cz-DiKTa show small fwhm values of 54, 47, and 53 nm, respectively. The small fwhm is correlated to the small Stokes shifts of 31− 50 nm, which indicates that the structural relaxation is small in their excited states. In contrast, the Stokes shifts of TMCz-DiKTa, DMAC-DiKTa, 3TMCz-DiKTa, and 3DMAC-DiKTa are much larger (86−242 nm), and the fwhm values are greater than 80 nm. These results indicate that in the presence of strong electron-donating groups, the SRCT character of the excited state disappears, and the long-range D−A type CT character starts to be dominant. These observations agree with the computational results shown in the Figure 2. We next probed how the nature of the emissive excited state evolves as a function of solvent polarity, and the results are presented in Figure S10. Cz-DiKTa, Cz-Ph-DiKTa, and 3Cz-DiKTa all show a small degree of positive solvatochromism that is characteristic of SRCT states associated with MR-TADF emitters. By contrast, TMCz-DiKTa, DMAC-DiKTa, 3TMCz-DiKTa, and 3DMAC-DiKTa show significant positive solvatochromism, suggesting that the lowest excited states of these compounds, especially in polar media, are LRCT in nature. They also show LRCT and SRCT dual emission in high polarity solvents, with emission moving from SRCT to LRCT with increasing polarity ( Figure S10). This was observed previously using a MR-TADF core (ADBNA-Me-Mes), with NMe 2 substitution. 19 The ΔE ST values were determined from the difference in energy of the onsets of the prompt fluorescence and phosphorescence spectra in toluene at 77 K. TMCz-DiKTa shows a broader emission spectrum but possesses an identical ΔE ST value to the predicted one, which indicates that there may be mixed SRCT/LRCT character in low polarity solvents such as toluene. The calculated D−A type emitters DMAC-DiKTa, 3TMCz-DiKTa, and 3DMAC-DiKTa show much smaller ΔE ST values, which reflects the smaller overlap integral.
We next evaluated the photophysical properties of the seven emitters in drop-cast 1,3-bis(N-carbazolyl)benzene (mCP) films at a doping concentration of 2 wt % (Table 1). This host was chosen due to its high triplet energy of 2.81 eV, 25 and the photoluminescence quantum yield (Φ PL ) was found to be the highest at a concentration of 2 wt % (Tables S5 and S6). Indeed, Cz-DiKTa, Cz-Ph-DiKTa, TMCz-DiKTa, DMAC-DiKTa, and 3Cz-DiKTa films show high Φ PL values of 90, 77, 71, 72, and 78%, respectively. However, 3TMCz-DiKTa and 3DMAC-DiKTa only present Φ PL values of around 20%, which may be attributed to the much stronger CT band and relatively lower calculated oscillator strength for the CT states (vide supra). Furthermore, their emission is red-shifted compared to the others, resulting in larger nonradiative decay processes. S 1 and T 1 levels in the doped film were determined from the onsets of the prompt fluorescence and phosphorescence spectra, respectively, measured at 77 K ( Figure S11). The corresponding ΔE ST values of Cz-DiKTa, Cz-Ph-DiKTa, and 3Cz-DiKTa are 0.14, 0.10, and 0.16 eV, respectively, which are slightly smaller than those measured in toluene glass. A possible explanation for the smaller ΔE ST in doped film can be attributed to the changes of conformation upon slow cooling of the film in comparison to flash freezing of the toluene glass samples, as well as host/guest interaction. 26 DMAC-DiKTa, TMCz-DiKTa, 3TMCz-DiKTa, and 3DMAC-DiKTa possess very small ΔE ST values ranging from 0.01 to 0.08 eV . Figure 4b,c show the time-resolved PL decays of the 2 wt % doped mCP films. All PL decays show prompt and delayed emission components at room temperature. For Cz-DiKTa, Cz-Ph-DiKTa, and 3Cz-DiKTa, the prompt emission life-   Organic Light-Emitting Diodes. We next fabricated vacuum-deposited OLEDs only with Cz-DiKTa, Cz-Ph-DiKTa, TMCz-DiKTa, DMAC-DiKTa, and 3Cz-DiKTa, as these compounds possessed suitably attractive Φ PL . Here we utilized a higher concentration of emitter (7.5 wt %) with the aim of improving the charge balance in the OLED device structure; we note that the photophysical behavior of the evaporated 7.5 wt % doped films in mCP is quite similar to that of the 2 wt % doped films in mCP, with only a small red-shift in the emission and a small decrease in Φ PL ( Figure S15 and Table S7). The optimized OLED structure is shown in Figure  5a    transporting layer, mCP acts as both the electron blocking layer and the host within the emissive layer, PPT acts as the hole blocking layer, TmPyPb acts as the electron transporting layer, and LiF acts as the electron injection layer. Figure 5e illustrates the EL spectra of these devices. The OLEDs with Cz-DiKTa, Cz-Ph-DiKTa, TMCz-DiKTa, DMAC-DiKTa, and 3Cz-DiKTa show electroluminescence maxima, λ EL , of 511, 492, 527, 549, and 547 nm, with corresponding Commission Internationale de l'Ećlairage (CIE) coordinates of (0.24, 0.61), (0.18, 0.50), (0.32, 0.60), (0.40, 0.57), and (0.39, 0.60), and the comparison between EL and PL of the doped films are shown in Figure S16. The devices based on the MR-TADF emitters, Cz-DiKTa, Cz-Ph-DiKTa, and 3Cz-DiKTa, show narrow electroluminescence spectra with fwhm values of 62, 61, and 54 nm which are slightly broader than that of DiKTa (39 nm). 19 The fwhm values are larger for the devices with TMCz-DiKTa and DMAC-DiKTa (78−89 nm), respectively. This reflects that the emission originates from a LRCT state in these devices in line with that observed in the PL spectra. Overall, the EL emission of the devices can be tuned from sky blue to yellow green by regulating the number and the electron-donating strength of the peripheral donor around the DiKTa core.
As depicted in Figure 5f and summarized in Table 2, the devices with Cz-DiKTa, Cz-Ph-DiKTa, TMCz-DiKTa, DMAc-DiKTa, and 3Cz-DiKTa show very high EQE max of 24.9, 23.0, 20.2, 23.8, and 24.4%, respectively, which is much higher than that of the DiKTa-based device with DiKTa (14.7%) reported by our group. 19 These EQE max values are among the highest reported for ketone-containing MR-TADF OLEDs (Table S8). Considering the measured Φ PL of the doped films fabricated by thermal evaporation (see Table S9) and assuming 25% outcoupling efficiency, the EQE max of these devices are expected to be, respectively, 19.3, 17.3, 14.3, 15.5, and 18.5%, which are much lower than the observed EQE max . One potential explanation would be if the transition dipole of the emitters were horizontally oriented parallel to the substrate surface, as this could lead to higher amount of the out-coupled light from the OLEDs to air. 2 We therefore measured the molecular orientations of evaporated doped films of the emitters, which are identical to the ones used in the OLEDs. The angle-dependent PL measurement results of these films are shown in Figure S19, and the anisotropy factors, a, extracted from the p-polarized emission were found to be 0.  Table S11). Combining the measured Φ PL of the films and the simulated outcoupling efficiencies, the EQE max values were expected to be 19.6, 16.8, 13.1, 14.3, and 18.8%, respectively, which are lower than the observed values. Therefore, the emitter orientation alone cannot explain this discrepancy. Similar higher than expected performance was also found in many other MR-TADF emitters. 22,27,28 Among these works, a group of structurally related emitters, QAD-Cz, QAD-2Cz, and QAD-mTDPA, reported recently by Zhang and co-workers, showed near unity Φ PL values of 99.6, 99.5, and 97.2% in mCP doped film, respectively, and EQE max of the corresponding OLEDs of 20.3, 27.3, and 23.9%, respectively, which implies outcoupling efficiencies of 20.4, 27.4, and 27.1%, respectively. 22 These EQE max values are also higher than 20%. Unfortunately, the anisotropy factors of these materials were not measured in the paper and the out-coupling efficiency was not discussed.
Although the origin of our higher-than-expected EQE max is not clear, we can envisage two potential causes (see the section "Out-coupling efficiency simulation and possible explanation about higher experimental EQE than predicted EQE" in the Supporting Information). The first is that the emission efficiency of our emitters might be underestimated due to some oxygen remaining in the integrating sphere during our measurements of the Φ PL ; also, in the OLED stack, emission efficiency can be enhanced by the Purcell effect. The second potential explanation could be the microcavity effects in the OLED stack leading to light emission that is directed forward more than for a Lambertian emitter and hence increasing the apparent EQE when measured in the forward direction. As the main focus of this work is to demonstrate the impact of donor substitution about DiKTa-type MR-TADF compounds and how it modulates the nature of the CT character of the emitters and affects the performance of the OLEDs, the origin of the apparently high out-coupling efficiency will be investigated in future work.
In addition to the high EQE max , these devices also show suppressed efficiency roll-off. The EQE values at 100 cd m −2 (EQE 100 ) for the Cz-DiKTa, Cz-Ph-DiKTa and 3Cz-DiKTa devices are 22.5, 19.2, and 17.8%, respectively, corresponding to an efficiency roll-off of 9.6, 16.5, and 27.0%, respectively. This performance is improved compared to the DiKTa-based OLED (44% reported by us 19 and 54% reported by Liao and co-workers). 27 The EQE 1000 values, however, drop dramatically with efficiency roll-off of between 50−74%, which is not uncommon in MR-TADF-based OLEDs such as Mes 3 DiKTa and DABNA-1. 8,19 Serious efficiency roll-off also was observed in the devices with QAD-Cz, QAD-2Cz, and QAD-mTDPA, where the EQE dropped to 0.73, 12.4, and 4.7% at 1000 cd/ m 2 , representing an efficiency roll-off greater than 55%. By contrast, for the devices with our D−A type TADF emitters, the EQE 1000 for the OLEDs based on DMAC-DiKTa and TMCz-DiKTa are 19.9 and 16.7%, these show a much smaller  (Figure 5c). The device performances reported in the present study are among the best results in ketone-containing MR-TADF devices (and devices containing a ketone-containing MR-TADF core as the acceptor in D−A emitters). Moreover, we demonstrate the importance of the choice of peripheral donor in order to maintain the MR-TADF character of the emitters.

■ CONCLUSIONS
In summary, through attaching different numbers of donors with different electron-donating strengths at the para position to the central nitrogen atom of the previous reported DiKTa core, the character of the charge transfer excited state can be modulated from SRCT to LRCT. This change in the nature of the emissive excited state is reflected in a broadening and a bathochromic shift of the emission. The photophysical properties, corroborated by SCS-CC2 calculations, show that the introduction of strongly electron-donating donor moieties to the periphery of the DiKTa core leads to a destabilization of the HOMO and an enhancement of the long-range CT character of the emitters. It is noteworthy that the SRCT character that is emblematic of MR-TADF compounds is conserved with the introduction of weak donors (Cz, Cz−Ph), so the color purity of these emitters is high. As a result, we achieved narrowband emission beyond 547 nm (fwhm = 54 nm) in the OLED accompanied by a high EQE max of 24.4% from the device with 3Cz-DiKTa. The Cz-DiKTa OLED exhibited the highest EQE max of 24.9% at λ EL of 511 nm. The OLED with D−A type emitter 3DMAC-DiKTa showed high EQE max of 24.3% and a small roll-off of 18.5% at 1000 cd m −2 . The strategy of judiciously decorating the MR-TADF core with weak donating groups is a useful tool to modulate the photophysical properties of these emitters and to realize highperformance OLEDs. However, too strong a choice of donor leads to the generation of donor−acceptor compounds, which leads to red-shifted and broadened emission in the device.

■ EXPERIMENTAL SECTION
General Method. HPLC analysis was conducted on a Shimadzu LC-40 HPLC system. HPLC traces were carried out using a Shimpack GIST 3 μm C18 reverse-phase analytical column. Melting points were measured using open-ended capillaries on an Electrothermal 1101D Mel-Temp apparatus and are uncorrected. High-resolution mass spectrometry (HRMS) was carried out at the BBSRC Mass Spectrometry Facility, University of St Andrews. Elemental analyses were carried out by Joe Casillo at the University of Edinburgh.
For emission studies, aerated solutions were bubbled by compressed air for 5 min and the degassed solutions were prepared via three freeze−pump−thaw cycles and spectra were measured using a homemade Schlenk quartz cuvette. Steady-state emission, excitation spectra and time-resolved emission spectra were recorded at 298 K using an Edinburgh Instruments FS5 fluorimeter. Samples were excited at 340 nm for steady-state measurements. Photoluminescence quantum yields for solutions were determined using the optically dilute method, 29 in which four sample solutions with absorbances of ca. 0.10, 0.075, 0.050, and 0.025 at 360 nm were used. The Beer− Lambert law was found to remain linear at the concentrations of the solutions. For each sample, the linearity between absorption and emission intensity was verified through linear regression analysis, with the Pearson regression factor (R 2 ) for the linear fit of the data set surpassing 0.9. Individual relative quantum yield values were calculated for each solution, and the values reported represent the slope obtained from the linear fit of these results. The quantum yield of the sample, Φ PL , can be determined by the equation  30 where A stands for the absorbance at the excitation wavelength (λ exc : 360 nm), I is the integrated area under the corrected emission curve, and n is the refractive index of the solvent with the subscripts "s" and "r" denote sample and reference, respectively. Φ r is the absolute quantum yield of the external reference quinine sulfate (Φ r = 54.6% in 1 N H 2 SO 4 ). 31 An integrating sphere (Hamamatsu, C9920−02) was employed for the photoluminescence quantum yield measurements of thin film samples. 32 The Φ PL of the films were then measured in air and N 2 environment by purging the integrating sphere with N 2 gas flow. Time-resolved PL measurements of the thin films were carried out using the time-correlated single-photon counting and MCS technique. The samples were excited at 375 nm by a pulsed laser diode (PicoQuant, LDH-D-C-375, fwhm < 40 ps, pulse energy = 58.5 ± 1.2 pJ, peak power = 1.5 ± 0.3 W, laser spot diameter = 0.4 ± 0.1 mm, power density = 11.6 ± 3.7 mW/cm 2 ) and were kept in a vacuum of <8 × 10 −4 mbar. The singlet−triplet energy splitting (ΔE ST ) was estimated by recording the prompt fluorescence spectra and phosphorescence emission at 77 K. The films were excited by a femtosecond laser emitting at 343 nm (Orpheus-N, model: SP-06-200-PP). Emission from the samples was focused onto a spectrograph (Chromex imaging, 250is spectrograph) and detected on a sensitivegated iCCD camera (Stanford Computer Optics, 4Picos) with subnanosecond resolution. Phosphorescence spectra were measured from 1 ms after photoexcitation, with an iCCD exposure time was 9 ms. Fluorescence spectra were promptly measured from 1 ns after photoexcitation with an iCCD exposure time was 99 ns.
Quantum Chemical Calculations. All ground-state optimizations were carried out using the DFT level with Gaussian09 33 using the PBE0 34 functional and the 6-31G(d,p) basis set. 35 Excited-state calculations have been carried out with the Turbomol/6.5 package for SCS-CC2 calculations. 15 Besides DFT calculations, we have investigated all compounds using spin-component scaled second order coupled-cluster (SCS-CC2). We first optimized the ground state using the SCS-CC2 method considering the cc-pVDZ basis set. 36 Vertical excited states were carried out on the ground state optimized structure using SCS-CC2 method. Different density plots were used to visualize change in electronic density between the ground and excited state and were visualized using the VESTA package. 37 Excited-state calculations also have been carried out using time-dependent DFT (TD-DFT) within the Tamm−Dancoff approximation (TDA) 38,39 with the same functional and basis set as for the ground state geometry optimization.
OLED Fabrication and Testing. OLED devices were fabricated using precleaned indium−tin oxide (ITO)-coated glass substrates with an ITO thickness of 90 nm. The OLED devices had a pixel size of 2 mm × 1 mm. The small molecules and cathode layers were thermally evaporated using a deposition chamber at 10 −7 mbar at 0.3 or 0.6 A/s for organic layers and 3 A/s for cathode. OLED testing was carried out using a Keithley 2400 sourcemeter and photodiode, assuming that the OLEDs show Lambertian emission. Electroluminescence spectra were collected using an Oriel MS125 spectrograph coupled to an Andor DV420-BU CCD camera.
Calculation of Out-Coupling Efficiency of OLEDs. Dipole orientation of emitter molecules was determined by angle-resolved PL measurements of thin films doped with each emitter. Our outcoupling simulation of the OLEDs is based on emission dipole as forced damped harmonic oscillator and embedded in thin film stacks.
Cartesian coordinates (XYZ) 1 H and 13 C NMR spectra, HRMS and HPLC of all target compounds; supplementary computational data; supplementary photophysical data(PDF)